The present invention relates to tone generators or contactless buzzers and in particular to a tone generator that is especially suited for use in automobiles for providing an audible as well as a visual warning indication of the occurrence of one or more predetermined conditions.
Current federal regulations require that all automobiles be equipped with devices that will provide a 4 to 8 second audible warning whenever the automobile is attempted to be operated without the seatbelts properly fastened. Such devices typically take the form of a buzzer unit that is controlled by a bimetallic timer circuit which is adjusted to time out within the appropriate time period. A warning light must also be activated contemporaneously with the buzzer, although federal regulations require that the audible warning operate despite an absence or failure of the warning light. Frequently, the same buzzer may also be used to provide a "headlamp ON" warning or a "key in the ignition" warning as well.
Although such devices are certainly adequate for their intended purpose, there are certain disadvantages that are inherent in any device that utilizes a bimetallic element. Bimetallic elements function to open a circuit by deflecting as the element is heated by the heating coil typically wrapped around the bimetallic element. However, in order to produce the desired time delay, the physical orientation of the bimetallic element must be manually adjusted so that the deflection of the element breaks the circuit at the appropriate time. In addition, the fine heating wire that is wrapped around the element usually must be spot welded rather than simply flow soldered along with the other components in the circuit. Thus, it is apparent that the manufacture of a buzzer unit having a bimetallic element is a rather labor intensive process, and therefore can add significantly to the basic component cost of the unit. Furthermore, in that a bimetallic element is a temperature responsive device, the time delay introduced by the element is affected by changes in ambient temperature. Accordingly, the accuracy of the unit can be seriously hampered if subjected to significant temperature variations. Moreover, the substantial heat generated by a bimetallic element can present problems for other circuit elements unless adequate heat dissipation is provided.
Accordingly, it is the primary objective of the present invention to avoid the disadvantages inherent in the use of bimetallic elements by designing a tone generator that utilizes positive temperature coefficient (PTC) resistors to generate the necessary time delay. The idea of substituting a PTC resistor for the bimetallic element is not new and has been attempted in the past. However, certain problems invariably arise which make the "straight-forward" substitution of a PTC resistor impractical. Specifically, because the resistance value of a PTC resistor is temperature sensitive, a straight series connection of a PTC resistor in circuit with a lamp and a logic line will not result in a circuit that will function adequately over the required temperature range. Moreover, if the lamp burns out or is removed, the change in the resistance characteristics of the circuit will prevent the PTC resistor from timing out. Furthermore, if a resistor is inserted in parallel with the lamp to insure the presence of a pull-down resistance in the event of a lamp failure, the additional voltage drop across the resistor will prevent the lamp from lighting properly. Consequently, because of these obstacles, PTC resistors have not been employed despite their apparent promise.
The present invention solves these problems by devising a circuit that utilizes a unique triple PTC element, with one PTC resistor used as the primary heating element, another as a switching element for the lamp, and the third as a logic switching element for the logic signal that times the tone generator. In this manner, the problems posed by the use of PTC resistors previously discussed are overcome. More particularly, since a separate logic switching element is provided apart from the heating element and the switching element that controls the activation of the lamp, the circuit will continue to function properly and provide an audible warning for the prescribed time period despite the absence of a bulb or a bulb failure. In addition, due to the utilization of a separate heating element, the delay time of the unit will remain within the 4 to 8 second specification period over a wide range of ambient temperatures. Importantly, the PTC element is inexpensive, does not require separate manual adjustment to set the delay time, and can be flow soldered to the printed circuit board along with the other circuit elements. Thus, a significant savings is realized in the cost of manufacturing a tone generator according to the present invention.
In addition, the present invention includes a novel electromagnetic transducer that is also inexpensive to manufacture. More particularly, a diaphragm is placed over a recess in the cover of the unit and secured by a foam gasket and plastic retainer ring that simply snaps into place, thereby forming a sealed sound cavity within the cover of the unit. A printed circuit board having the electrical components of the control circuit and the electromagnetic coil mounted thereon, is secured between the cover and the casing of the unit when the outer two pieces are joined together. The relative position of the coil and the diaphragm when the unit is assembled is designed so that the proper air gap will result between the pole piece of the coil and the diaphragm, thus eliminating the need for manually setting the appropriate air gap.
Further objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which: